Tadpole shrimp structure macroinvertebrate communities in playa ...

More documents

Recommendations

Info

146 removed. Hence, one might expect their abundances to be positively correlated with tadpole shrimp density across control microcosms. However within control trials, moinid density was negatively correlated with tadpole shrimp density (Fig. 2). Diaptomid abundance in microcosms was marginally correlated with tadpole shrimp density in one control trial (r ¼ )0.296, p ¼ 0.060). The presence of tadpole shrimp may be beneficial to these taxa, but such benefits decrease with increasing tadpole shrimp density. Discussion The macroinvertebrate composition of playa microcosms was highly variable within treatment groups, probably as a result of subtle differences in hatching cues among replicates, lack of perfect homogeneity in soil samples, natural variation in hatching rates for organisms, playa-specific differences in the taxonomic composition of innocula, and our ability to remove tadpole shrimp from treatment microcosms. Even within the statistical constraints imposed by such variability, it was clear that tadpole shrimp played an important role in structuring these communities. The presence of tadpole shrimp was associated with reduced abundances of ostracods, Culicidae, and Psychodidae. Moinidae, Diaptomidae, Streptocephalidae, and Sminthuridae were positively affected by the presence of tadpole shrimp, and microcosms with tadpole shrimp had higher taxonomic richness and diversity. The importance of tadpole shrimp on playa microcosm communities is a consequence of their dual role as predators and ecosystem engineers, perhaps analogous to the effects that rodents have on desert plant communities through seed predation and burrowing activities (Moorhead et al., 1988; Brown & Heske, 1990; Willig & McGinley, 1999). Tadpole shrimp are opportunistic predators that have been shown to decrease abundances of prey species (Dodson, 1987; Fry et al., 1994; Walton, 2001). As tadpole shrimp increase in abundance, the total number of prey consumed increases, leading to negative correlations between many taxon abundances and tadpole shrimp in control trials. Predation may be accelerated as tadpole shrimp broaden their diet as detritus becomes scarce. In addition, tadpole shrimp may have indirect positive effects on community diversity by preferentially preying on certain taxa, giving rarer competitors a better chance of survival (i.e., predator-mediated coexistence) and leading to increased taxonomic richness. Indeed, early stages of invertebrate community development in playas are dominated by detritivores (Moorhead et al., 1998), that may compete for resources. In addition to predation, tadpole shrimp have foraging behaviors that may affect community structure. Tadpole shrimp feed by sifting through benthic debris and sediments. This may lead to direct competition with taxa, such as ostracods, which also tend to feed along the sediment surface (Delorme, 1991). Such competition could be responsible for negative correlations between many taxa and tadpole shrimp abundances in control trials. The foraging activities of tadpole shrimp may also be responsible for increased abundances of many taxa, increased richness, and increased diversity in the presence of tadpole shrimp. Tadpole shrimp scatter detritus as they feed, which floats to the water surface potentially benefiting detritivorous invertebrates, such as collembola, which feed along the surface film, and fairy shrimp, which filter the water column (Christiansen & Snider, 1984; Hilsenhoff, 1991). Moreover, the agitation of benthic sediments may increase the hatching rate of other invertebrates, such as cladocerans and clam shrimp, which rely on changes in light or oxygen concentration as hatching cues (Belk & Cole, 1975; Wiggins et al., 1980). Microcosm communities undoubtedly differ from natural playa lakes (Anderson & Smith, 2004). Effects of tadpole shrimp on resident macroinvertebrate communities in microcosms may be representative of their effect during early succession. Overwintering residents such as phyllopod crustaceans generally dominate early successional stages of playa invertebrate communities (Sublette & Sublette, 1967; Moore, 1970; Lake et al., 1989). Consequently, tadpole shrimp may exert strong influences on invertebrate communities during the first few weeks of playa inundation when relative abundances of insect predators and other immigrants are low (Lake et al., 1989; Schneider & Frost, 1996; Moorhead et al., 1998). The composition of early resident
communities may influence later stages of succession which transpire when conditions are more favorable for transient species (Lake et al., 1989). Particularly, the presence of tadpole shrimp may reduce the establishment of mosquito populations (Tietze & Mulla, 1991; Fry et al., 1994), although this is difficult to ascertain as Culicidae were rare in experimental microcosms. Whether early successional effects of tadpole shrimp are strong enough to influence macroinvertebrate communities in playa lakes remains to be seen. Invertebrate diversity in playas of West Texas are affected by a wide range of local factors, including period of inundation, basin modification by agricultural practices, and vegetation (Sublette & Sublette, 1967; Rhodes & Garcia, 1981; Bolen et al., 1989; Haukos & Smith, 1994; Hall et al., 1999; Hall et al., 2004). Furthermore, the effects of a single species, such as tadpole shrimp, on community structure likely depends on many contextual factors such as productivity (Balciunas & Lawler, 1995; Leibold, 1996), disturbance (Menge et al., 1994), and community composition (Power et al., 1996). Acknowledgements We thank B. Croyle, L. Peppers, and A. Shaner for assistance with laboratory work, and D. Hall for assistance with invertebrate identification and collection. We also thank the landowners who allowed access to their playas. G. Dwyer, D. Yee, and reviewers provided helpful comments on an earlier draft of this manuscript. Research was funded by grants to M. R. Willig, D. L. Moorhead, R. B. Sites, T. R. Mollhagen, and E.B. Fish from the Environmental Protection Agency (EMAP-01-93) entitled ‘‘Integrated indicators of stress in playa lakes: wetland ecosystems in a sea of aridity and agriculture’’ and to L. Blanton and J. M. Burns from the Howard Hughes Medical Institute through the Undergraduate Biological Sciences Education Program. References Abrams, P. A., B. A. Menge, G. G. Mittelbach, D. A. Spiller & P. Yodzis, 1996. The role of indirect effects in food webs. In Polis, G. A. & K. O. Winemiller (eds), Food Webs: Integration of Patterns and Dynamics. Chapman & Hall, New York: 371–395. 147 Anderson, J. T. & L. M. Smith, 2004. Persistence and colonization strategies of playa wetland invertebrates. Hydrobiologia 513: 77–86. Ardo, P., 1948. Some notes on phyllopods in temporary pools of the Alvar of Oland in South Sweden. Lunds Universitets Arsskrift 44: 1–22. Balciunas, D. & S. P. Lawler, 1995. Effects of basal resources, predation, and alternative prey in microcosm food chains. Ecology 76: 1327–1336. Belk, D. & G. A. Cole, 1975. Adaptational biology of desert temporary pond inhabitants. In Hadley, N. F. (ed), Environmental Physiology of Desert Organisms. Dowden, Hutchinson & Ross Inc., Stroudsburg, PA: 207–224. Black, R. W. & N. G. Hairston, 1988. Predator driven changes in community structure. Oecologia 77: 468–479. Bolen, E. G., L. M. Smith & H. L. Schramm, 1989. Playa lakes: prairie wetlands of the Southern High Plains. Bioscience 39: 615–623. Brooks, J. L. & S. I. Dodson, 1965. Predation, body size, and composition of plankton. Science 150: 28–35. Brown, J. H. & E. J. Heske, 1990. Control of a desert-grassland transition by a keystone rodent guild. Science 250: 1705–1707. Chapin, F. S., III, B. H. Walker, R. J. Hobbs, D. U. Hooper, J. H. Lawton, O. E. Sala & D. Tilman, 1997. Biotic control over the functioning of ecosystems. Science 277: 500–504. Christiansen, K. A. & R. J. Snider, 1984. Aquatic Collembola. In Merritt, R. W. & K. W. Cummins (eds), An Introduction to the Aquatic Insects of North America. Kendall/Hunt Publishing Company, Dubuque, IA: 82–93. Christoffersen, K., 2001. Predation on Daphnia pulex by Lepidurus arcticus. Hydrobiologia 442: 223–229. Clarke, K. R. & R. M. Warwick, 1994. Change in Marine Communities: an Approach to Statistical Analysis and Interpretation. Nature Environment Research Council, U.K. Delorme, L. D., 1991. Ostracoda. In Thorp, J. H. & A. P. Covich (eds), Ecology and Classification of North American Freshwater Invertebrates. Academic Press, San Diego, CA: 691–722. Dodson, S., 1974. Zooplankton competition and predation: an experimental test of the size efficiency hypothesis. Ecology 55: 605–613. Dodson, S., 1987. Animal assemblages in temporary desert rock pools: aspects of the ecology of Dasyhelea sublettei (Diptera: Ceratopogonidae). Journal of the North American Benthological Society 6: 65–71. Dodson, S. & D. Frey, 1991. Cladocera and other Branchiopoda. In Thorp, J. H. & A. P. Covich (eds), Ecology and Classification of North American Freshwater Invertebrates. Academic Press, San Diego, CA: 723–786. Fry, L. L., M. S. Mulla & C. W. Adams, 1994. Field introductions and establishment of the tadpole shrimp, Triops longicaudatus (Notostraca, Triopsidae), a biological-control agent of mosquitos. Biological Control 4: 113–124. Hall, D. L., R. W. Sites, E. B. Fish, T. R. Mollhagen, D. L. Moorhead & M. R. Willig, 1999. Playas of the Southern High Plains: the macroinvertebrate fauna. In Batzer, D. P., R. B. Rader & S. A. Wissinger (eds), Invertebrates in Freshwater Wetlands of North America: Ecology and Management. Wiley, New York: 635–665.
Page 1 and 2: Hydrobiologia (2005) 541: 139-148
Page 3 and 4: trials, tadpole shrimp were removed
Page 5 and 6: Table 2. Mean and standard error (S
Page 7: Figure 1. Means and standard errors

Tadpole shrimp structure macroinvertebrate communities in playa ...

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?